Centrifugal pumps for petroleum production are made up of a large number of stages. Each stage has an impeller that is rotated by a shaft driven by an electrical motor. Each impeller is located within a stationary diffuser. Each diffuser has passages that extend downstream and radially inward toward the shaft for receiving fluid from an upstream impeller and delivering the fluid to a downstream impeller. Each impeller has a central inlet and passages that extend outward in a downstream direction for delivering well fluid to a downstream diffuser.
The rotation of the impeller causes down thrust. Typically, each impeller is free to float axially on the shaft, and transmits the down thrust to its mating diffuser. Furthermore, thrust washers are located between the mating surfaces for handling the rotating sliding engagement between the impeller and the diffuser.
One type of thrust washer is made of phenolic material, which is not particularly hard. Another type, which is used for abrasive well fluid conditions, is of a hard, wear resistant metal such as tungsten carbide. The diffuser and impeller are cast of a metal such as Ni-Resist. Normally, the thrust washer is attached to the impeller for rotation therewith, such as by adhesive or by an interference fit. One problem with adhesive is that the bonding surface of the impeller must be very clean and free of oil. Also, the adhesive has to have time to cure. Further, in high temperature wells, the temperature may exceed that of the adhesive, causing it to deteriorate. If the thrust washer begins to spin relative to the impeller, damage to the impeller may occur.
An interference fit requires a high tolerance for the mating components. Also, it may not be as reliable as the adhesive because variations in the force fit installation. The differences in the coefficient of expansion of the impeller and a tungsten carbide thrust washer could cause the thrust washer to become loose at high temperatures. An interference fit required to hold a tungsten carbide thrust washer at high temperatures may be so large that the thrust washer fractures during assembly.
The diffuser has an internal bearing support that receives a bearing sleeve for engaging the rotating shaft. The bearing sleeve is typically installed in the bearing support by heat shrink and force fit techniques. In high temperature operations, the differences in thermal expansion of the bearing sleeve can cause the bearing sleeve to become loose and fall out or to spin in the bearing holder of the diffuser. Force fits may not be successful when the plastic deformation of the bearing holder material of the diffuser causes the bearing to become loose at high temperatures. An interference fit required to hold the bearing sleeve at high temperatures may be so large that the bearing fractures during assembly.